Numerous methods are known for the detection of compounds containing thiol groups. A method which is mentioned particularly frequently is that described by G. L. Ellman in Arch. Biochem. Biophys., 82:70-77 (1959). The described method is based on the formation of the yellow anion of 3-mercapto-6-nitro-benzoic acid formed by reaction between a thiol and 3,3'-dithio-bis-6-nitro-benzoic acid (Ellman's reagent).
The reaction is sensitive and fast. However, the anion of Ellman's reagent is very sensitive towards oxidation and the coloration formed is therefore unstable. Furthermore, Ellman's reagent is very sparingly soluble in polar solvents, such as, for example, water and, therefore, can be used to only a limited degree for some thiol solutions, especially those which are prepared biochemically in aqueous systems.